Can someone explain the benefit of the SSGI addon for Eevee?
TLDR: nice light bounces but faster than Cycles.
As you are aware EEVEE is a much faster render engine than Cycles. It builds on the approach of rasterization instead of raytracing. This means that it basically tries to calculate the color value for each pixel more directly, while raytracer like Cycles is taking an approach of simulating the approximate movement of light through the scene.
This leads to a variety of advantages and shortcomings. One of this shortcomings is that the dogmatic rasterization approach doesn't allow for bounce light (as this falls more into the simulation realm).
This addon works around that particular shortcoming. For example if you are dependent on the speed of EEVEE, can live with most of its shortcomings, but still need light interaction between your objects and can't bake your whole light because you for example have movement in your scene, than this is a quick way to achieve this.